Implement `KeyboardEvent.GetModifierState`

[stroiman]

GetModifierState

• In Go, this should exist on the EventInit - Probably ModifierEventInit
• In JavaScript, this should exist on KeyboardEvent

See documentation of how "events" are implemented: https://github.com/gost-dom/browser/blob/main/docs/Events.md
MDN Docs: https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/getModifierState

"Good first issue" maybe a bit of a stretch, as at least the JavaScript method should be code generated.